1. Field of the Invention
The present invention relates to a power amplifier, and in particular, to a power amplifier whose structure is reduced in size.
2. Description of Related Arts
Two kinds of output control systems have been employed for the conventional power amplifiers. In an output control system, as shown in FIG. 1, bias voltages of linear amplifiers 3, 4 and 5 are kept constant to keep their gains constant, and programmably variable gain amplifier 2 is arranged before input terminal 6 of power amplifier 1, and the magnitude of an input signal applied to power amplifier 1 is changed by variable gain amplifier 2. In the other system, as shown in FIG. 2, the gain of power amplifier 1 is changed by controlling the bias voltage of linear amplifiers 3, 4, and 5.
In the amplification mode of the conventional power amplifier described above, in particular, in an amplitude modulation mode and a phase modulation mode, since the data are transmitted by changing of amplitude component, a distortion produced during the compression of the data by the power amplifier, deteriorates the quality of the data. Therefore, the power amplifier needs to be used in a linear amplification range, which reduces the efficiency of the power amplifier. This is a serious matter to induces a problem that the power amplifier itself or the control system therefor needs to be selected according to the modulation mode of signal.